The present invention relates to an optical amplification and/or light emitting element. More specifically, the present invention relates to an optical amplification and/or light emitting element such as a surface-emitting element or a surface-emitting amplifier that comprises a medium having the function of light emission or optical amplification, including lasing, and a lasing structure that surrounds that medium.
A surface-emitting element or optical amplification element in the prior art is excited by current injection directly through an active medium. A typical example is a configuration in which minority carriers are injected into a hetero-junction that means a kind of a p-n junction.
Conceptual views of the basic structure of a prior-art surface-emitting element are shown in FIGS. 5A and 5B. In other words, FIG. 5 is a plan view thereof and FIG. 5B is a section taken through the vicinity of the center thereof. The element shown in the figures is a vertical cavity surface-emitting laser (VCSEL) of a structure that will be described below by way of the fabrication process thereof.
First of all, a highly reflective distributed Bragg reflector (DBR) 2 that is a multi-layer structure of quarter-wavelength-thick InAlAs layers and InP layers is grown on top of an n-type InP substrate 1. A cladding layer 3 of n-type InP and an active layer 4 are then grown thereupon. The active layer 4 could have a strained multiple-quantum well (MQW) structure consisting of alternate layers of two types of InGaAsP materials having mutually different compositions, by way of example. A cladding layer 5 of p-type InP is then grown, followed by a highly reflective DBR 6 that is a multi-layer structure of quarter-wavelength-thick InAlAs layers and InP layers.
The DBR on the upper side is then etched to form a cylindrical mesa 10. The InAlAs layers within the DBR 6 on the upper side are selectively oxidized from the sides of the cylindrical mesa 10 to convert them into selectively oxidized layers 15. These selective oxidation layers 15 function as blocking layers for constricting the current in the active layer 4. The selective oxidation layers 15 have a refractive index that is significantly less than those of the InAlAs layers and InP layers of the DBR 6, making it possible to increase the reflectivity of the top DBR 6.
Each selective oxidation layer 15 acts as an insulating body, which means that no current will flow in the active layer 4 if the selective oxidation should happen to extend throughout the entire cylindrical mesa 10. The progress of the selective oxidation must therefore be halted at an optimal distance from the side walls of the cylindrical mesa 10. If the progress of the selective oxidation layers 15 is insufficient, the reflectivity of the current constriction region will rise, which will decrease the lasing efficiency. If the progress of the selective oxidation goes too far, on the other hand, the current constriction region will be too narrow, increasing the element resistance. To start with, the passage of a current through an ultra-small active region is itself disadvantageous from the practical point of view, in that the current density is likely to increase and the temperature is likely to rise.
It is necessary to have an electrode 20 on the upper surface of the circular cylindrical mesa 10, to induce a current therein, but this electrode 20 must be transparent to ensure that light can emit therethrough. An annular electrode with a circular window for light output is also necessary.
This VCSEL of the prior art has many problems from the characteristics point of view and the manufacturing point of view. In general, any surface-emitting element that requires a current injection therethrough has, more or less, problems that are similar to those of a VCSEL.
The present invention was devised to solve the above described technical problems. In other words, an objective thereof is to provide a high-performance, optically pumped, surface-emitting type of optical amplification and/or light emitting element that can also be manufactured extremely efficiently.
In accordance with the present invention, a medium having the function of light emission or optical amplification induced by optical pumping is surrounded by an annular laser (ring laser) having a diffraction grating (holographic structure) of the second order or more. The output from the ring laser comes via this diffraction grating (holographic structure) as radiation mode. This is used for optically pumping up the medium at the center of this structure. To stabilize the longitudinal mode of the ring laser with greater radiation mode output, a gain medium and a reflective structure could be provided around the periphery of this ring laser as an additive structure. In other word, this element also plays a role of optical pumping from the sides of a VCSEL.
In other words, an optical amplification and/or light emitting element in accordance with the present invention comprises:
a medium capable of at least light emission or amplification by optical pumping; and
a ring waveguide of at least one turn, formed so as to surround the medium;
wherein the waveguide comprises a holographic element for extracting at least part of the light that is guided by the waveguide and emitting towards the medium.
In this case, the holographic element could be a diffraction grating of the second order or more, by way of example.
As a further aspect of the present invention, the medium is bounded vertically by a pair of reflective mirrors that are substantially parallel to the predetermined plane surface.
As a further aspect of the present invention, the medium and the pair of reflective mirrors form a VCSEL structure.
As a further aspect of the present invention, the optical amplification and/or light emitting element is annularly provided with another gain medium alongside of the ring waveguide.
As a further aspect of the present invention, the optical amplification and/or light emitting element is annularly provided with a phase adjustment mechanism alongside the ring waveguide.
As a further aspect of the present invention, the optical amplification and/or light emitting element is annularly provided with high reflection structure on the outer side of the ring waveguide.
As a further aspect of the present invention, the waveguide comprises an active layer in which gain is produced by current injection.
As a further aspect of the present invention, the ring waveguide is a ring laser.
As a further aspect of the present invention, the medium comprises a fluorescent material and the waveguide is a ring laser operating at blue or violet or ultraviolet wavelengths.
As a further aspect of the present invention, the optical amplification and/or light emitting element is further provided with a second medium that is capable of controlling the absorption index, between the medium and the waveguide, and the strength of pumping light that is incident on the medium can be modulated.
As a further aspect of the present invention, the medium, the waveguide, and the holographic element are integrated in a monolithic structure.
The present invention makes it possible to cause efficient lasing of an ultra-small optical amplification and/or light emitting element that is capable of optical pumping. In particular, when this invention is applied to elements such as VCSELs, the current pumping is limited to the ring laser for pumping, which has a comparatively large volume, making it unnecessary for current to flow in the VCSEL. It is therefore simple to fabricate, with no worries about any trade-off between blocking by the electrode and light extraction.
Since no current flows in the VCSEL, heat generation by the VCSEL itself is suppressed. This results in significantly improved temperature characteristics and an improvement in reliability. At the same time, since no current flows through the VCSEL, it is possible to form the MQW structure that is the active layer thereof to be far thicker than in the prior art. As a result, the gain can be greatly increased.
Furthermore, the element of the present invention has a planar disposition that is designed for monolithic integration with other electrical and optical components.